Black tank flush system has an 'anti-siphon vacuum breaker backflow preventer' at the top of that line that comes from the hose connection.
Then a line comes out the side of that 'device' and goes down to the spray nozzle that is installed in sidewall of black tank
Either the 'device ' is bad OR more likely is the nozzle is clogged
The device looks something like this:
Can be black plastic, white/clear plastic or brass
And are typically in cabinet under bathroom sink